A nanosized material shows different electrical, optical and/or magnetic properties, as compared to bulk materials and, therefore, has recently attracted attention in a wide range of applications. Especially, a substance in a nanotube structure having nanosized holes has relatively high specific surface area and a number of researches have been conducted for structures of various substances. In addition, since the nanotube structure can be multiply used in a variety of applications with respect to modern sciences and technologies, a great deal of studies and investigations into the same are currently conducted over the world.
As such, a nanosized material, in particular, an atomic structure and/or constitutional composition thereof closely associates with a structure and/or characteristics of a resultant nanosized substance. Therefore, in order to obtain a final nanosized product having desired structure and characteristics, an investigation into control of such atomic structure and/or natural features of a raw nanosized material must be preceded.
However, it is difficult to fabricate nanosized substances having desired structure and/or characteristics by controlling the structure in atomic scale and development of such techniques is now slightly progressed. As for a TiO2 bulk material, an anatase phase is mostly studied for its application to a light absorbing material, and an anode of a lithium secondary cell, etc., and is known to have a wide band gap energy of 3.2 eV capable of absorbing UV ray ranges only. In order to improve light absorption efficiency of the anatase phase of the TiO2 bulk material, efforts for control of the band gap energy by improving an electronic structure thereof have been continued. As an method for improving the electronic structure, a process for doping different elements on an anatase phase of the TiO2 bulk material was proposed as the most preferable and simple technique. Especially, it has been reported that doping of nitrogen among such different elements may considerably reduce a band gap energy sufficiently to extend a light absorption layer of TiO2 up to a visible light area. Nitrogen doping into TiO2 may induce adsorption of nitrogen in molecular state, penetration of nitrogen into a TiO2 matrix, or substitution of oxygen with nitrogen. It was also reported that the nitrogen doping in the form of oxygen substitution with nitrogen is most effective to control an electronic structure of TiO2. Method for doping nitrogen may include, for example: a method for doping nitrogen by adding nitrogen to gas or a solution during formation of a thin film or a nanosized structure of TiO2; and, as a post processing, a method for directly penetrating nitrogen ions into TiO2 by thermal treatment or ion implantation under nitrogen atmosphere, and so forth. However, these techniques generally have a drawback such that nitrogen is not doped in atomic state, instead of being adsorbed in molecular state. In addition, nitrogen ions directly injected and penetrated into a TiO2 matrix often cause a damage to an original structure of TiO2, thus entailing difficulty in maintaining an initial anatase phase of the TiO2.
Since a TiO2 nanotube typically has a higher specific surface area and excellent vertical orientation than TiO2 bulk material, relatively large reaction area and/or high electron mobility may be expected. Accordingly, if substitution of atomic nitrogen is done on the TiO2 nanotube, an electronic structure and a band gap of the nanotube are controlled so that enhanced performance of the TiO2 nanotube may result more than light or electrochemical application of the nanotube is performed.
Examples of conventional technologies relating to the present invention may include technical concepts disclosed by A. Ghicov (A. Ghicov, J. Macak et al., NanoLett. 2006. 6. 1080) and R. P. Vitiello (R. P. Vitiello, J. M. Macak et al., Electrochem. Commun. 2006. 8. 544), however, these techniques encountered problems in that nitrogen is not doped in atomic state, instead of being adsorbed in molecular state, and nitrogen ion irradiation causes significant breaking of the original TiO2 structure.